In conventional display with three primary colours, two types of digital devices for displaying images by projection are known:
the projectors having a microimager for each primary “channel”: for the display of an image (generally of the LCD or LCOS type), each microimager then forms a component image of this image to be displayed, a first one of red hue, a second one of green hue and a third one of blue hue; these three component images form the same series for the display of this image; the various microimagers are imaged by an optical system on the same screen in such a way that the component images of this same series are displayed simultaneously on the screen to form the image to be displayed; this is the first conventional display method of the prior art;
the projectors having a single microimager for all the primary “channels” (generally of the “DMD”, i.e. “Digital Micromirror Devices”, type); these projectors generally have a coloured wheel (or an equivalent device) which enables the single microimager to be sequentially illuminated using each display primary colour. The single microimager then successively forms each component image: a first one of red hue, a second one of green hue and a third one of blue hue; each component image forms a different series; the microimager is imaged by an optical system on a projection screen in such a way that the component images of each series are successively displayed on the screen to form, by colour fusion in the observer's eye, the image to be displayed; this is the second conventional display method of the prior art. This successive display then produces a flicker to which the eye can under certain circumstances be sensitive. The eye then perceives a colour break-up defect which can be disturbing. It is noted that this artefact is indeed all the more visible as the contour on which it appears is sharp, i.e. that it contains high spatial frequencies. These contours are located, by definition, in image transition zones; conversely, the transition zones with low spatial frequency generally, but not always, reflect the absence of contours.
The display method according to the invention therefore uses the second display method of the prior art and, optionally, in addition the first one; the projector therefore has a microimager for each series of component images. One purpose of the invention is to reduce the colour break-up defect.
It is known that the human vision has very particular characteristics for the perception of the spatiotemporal contrasts. In particular, these differ according to the eye retina illumination. The retinal illumination is measured in particular in trolands, where 1 Tr is 1 Cd/m2 for an opening of the eye pupil of 1 mm2. For a given retinal illumination and a constant flicker amplitude, the perception that a human observer will have of a temporal and/or spatial oscillation for a first wavelength, for example around 450 nm (blue), will not be the same as the one he may have of this same oscillation for a second wavelength different from the first one, for example 520 nm (green).
Examples drawn from the literature on the eye sensitivity variations according to the contrast temporal frequencies and/or contrast spatial frequencies (vertical and/or horizontal oscillations) are illustrated in the following figures:
FIG. 1: according to the temporal oscillation frequency expressed in cycles per second (“cps” or Hertz) and to the retinal illumination for a white light and a null spatial oscillation frequency;
FIG. 2: according to the spatial oscillation frequency (expressed in cycles per degree (“cpd”) and to the wavelength expressed in the form of colour hues, for a constant retinal illumination and a null temporal oscillation frequency;
FIGS. 3 and 4: according to the temporal oscillation frequency expressed in Hertz and to the spatial oscillation frequency expressed in cycles per degree (“cpd”), for a white light (“achromatic”) and a constant retinal illumination.
According to these last two figures, it is noted that the sensitivity to the temporal contrast decreases with the frequency of the spatial oscillations, as soon as the frequency is less than 8 cpd and 10 Hz approximately.